A union is another type of class of the C++ language. Like
the other two, it is created from combining other
identified variables. To create a union, use the union keyword following
the same syntax applied for a structure, like this:

union UnionName {};

Like another class, the members of a union are listed in its
body. For example, to create an object that represents a student, using a union,
you could define it as follows:

When creating a class of struct or class type, each variable, treated as its own
entity, occupies its assigned amount of space in memory. This is different with
unions. All of the members of a union share the same memory space. After
declaring a union object, instead of allocating memory for each member variable,
the compiler assigns an amount of memory equal to the largest variable. If you define a union that would represent a
CHouseAge object as the above, the compiler would assign the same memory space to
all members; and the compiler would find out which member needs the largest
space. In our example, the Age member variable uses more memory than the
YearBuilt. Therefore, the compiler will allocated the amount of memory required
by a double, then all member variables would share that memory. Consequently, a union can store the value of only one of its members at a
time: you cannot simultaneously access the values of all members, at the same
time. If you initialize all of the members of a union at the same time, the
result is unreliable; since all members are using the same memory space, that
space cannot accommodate all member values at the same time.

Because only one member of a union can be considered for an
instance of the object, use a union you are in an "either of one"
scenario. For example, when processing our CHouseAge object, we want Either the
year the house was built OR its age. We don't need both values. In fact this
would create redundancy in the program. Notice that, if we know the year the
house was built, we can deduct its age. On the other hand, if we know the age of
the house, we can calculate the year it was built.